This invention relates to the synthesis of signals from stored data which are sets of the coefficients of a recursive filter where each set is applied to the filter for a fixed time period, the periods totaling the duration of the synthesized signal. More particularly, the sets of coefficients include the probabilistic modeling of non-repetitive ensembles of transient sounds by means of Markov transitions over sets of vector quantized linear predictive coding (LPC) modeling coefficients.
In patent application U.S. Ser. No. 332,414, now U.S. Pat. No. 5,005,204, issued Apr. 2, 1991, assigned to the same assignee as this application, which is incorporated herein by reference, a single analog sound transient was divided into time-blocks of equal length and for each block a set of coefficients for a linear predictive coding (LPC) circuit was obtained. The circuit provided a satisfactory replica of the signal from which the LPC modeling coefficients were obtained with a small variation in the sound of successive synthesized signals because of the randomness produced by the random noise source applied to the input of the linear predictive filter.
The modeling of non-stationary acoustic signatures, termed transients, is of critical importance in the design of detection processors and in the accurate production of synthetic signatures for sonar trainers, and test target generators. Sonar training, which involves the recognition of a contact's existence and maneuvering condition, requires an accurate interpretation of transient contact emissions which in turn must be faithfully generated by a contact synthesizer. Currently such sounds are provided by magnetic tape players which are cumbersome to use, are not synchronized with the evolving scenario, and produce distorted results due to recorded background noise. Transient detection processing which provides the means for identifying the otherwise quiet contacts, requires a transient signature model which not only is of high-fidelity, but, in addition, provides the basis for detection methods which are processing efficient.
Classified as transients are those signatures of possibly several hours duration which exhibit a rapid, yet statistically regular, pattern of spectral energy fluctuation. Examples of the sounds to be modeled include hatch closings, shrimp snaps, ice crackling, hull groans, shifting cargo, and steering actuator emissions.
As a result of LPC analysis/synthesis, a reasonable replication of the original sound is attained. The required synthesis is accomplished by a recursive filter as shown in FIG. 2 of the referenced application. The arithmetic burden required for the implementation of the synthesis is significant. Each output sample requires (N+1) multiply-accumulations where N is the number of filter coefficients. If the output data rate is F.sub.S samples per second, then (N+1)F.sub.S multiply-accumulations per second are required. An even more significant factor from an implementation viewpoint is the total number of modeling filter coefficients required for synthesizing a signature. These coefficients must be stored in a limited size synthesizer memory and therefore constitute a significant implementation burden. Every block of 8 millisecond duration, for instance, requires approximately ten coefficients and one gain value. The resulting number of bits of coefficient memory for LPC synthesis is thus, N.sub.c =(16)(125)(11)T.sub.s =22000 T.sub.s bits of memory, where 16-bit words are used, 125 blocks per second are processed, 11 words per block are required, and T.sub.s is the total duration of signature synthesis in seconds. For direct synthesis from stored sample at an assumed 16 kHz sample rate, the required number of bits is N.sub.d =(16)(1.6.times.10.sup.4)T.sub.s =256,000 T.sub.s bits so that a 10:1 data compression is accomplished. However, it can be seen that one megabit of RAM memory will support only 45 seconds of LPC synthesized sound. The synthesis requirement exists for non-stationary sounds which last for several hours, so, clearly, additional data compression is desirable.